Or as Gilad Bracha summarized (in reference to Cook's article): "The notion of an interface is what truly characterizes objects - not classes, not inheritance, not mutable state."

https://gbracha.blogspot.com/2022/06/the-prospect-of-executi...

(the "not mutable state" bit is what makes a function an object for me but that's an ancient debate: http://people.csail.mit.edu/gregs/ll1-discuss-archive-html/m...)